1. Field of the Invention
The present invention relates to a driver stage, and particularly to adjusting the output impedance of a driver stage for use in data transmission between semiconductor components.
2. Description of the Related Art
In data transmission between semiconductor components, the impedance properties of an interconnection line may have significance, specifically at high data rates. If an output resistance of a transmitter and/or the input resistance of a receiver are not matched to a line impedance, reflections arise at the line ends and overshooting and undershooting occur. As a result, the data transmission may not be accurate, and the signal falsified. Terminal resistors that are matched as accurately as possible to the line impedance may be integrated in transmitter and receiver modules. However, resistors based on polysilicon integrated in semiconductor components may have a resistance that fluctuates depending on process properties, temperature, and the supply voltage.
FIG. 7 shows diagrammatically a model for the data transmission between two semiconductor components including a transmitter 100 and a receiver 140. The connection between the semiconductor components 120 is effected by two lines, both characterized by a line impedance Z. The transmitter 100 has a differential output with a positive data output DP and a negative data output DM, corresponding to the two lines. The respective output resistors of the transmitter 100 are shown diagrammatically by R1, while the respective input resistors of the receiver 140 are shown diagrammatically by resistors R2.
FIG. 8 shows a typical configuration of the drive stage of a transmitter 100, such as the transmitter 100 of FIG. 7. The transmitter 100 has a pre-driver stage 11, configured to convert an input signal from an individual line into a differential signal for transmission via the two lines. One of two output driver stages 12, 14 is associated with each of the two signal components of the differential signal.
The output driver stages 12, 14 each include an inverter having a series connection of a pull-up transistor 15, two resistors 18 and a pull-down transistor 16 connected in sequence between a positive voltage source and a negative voltage source. The positive voltage source may be a positive supply voltage and the negative voltage source may be a ground. The pull-up transistor 15 is configured as a PMOS transistor, and the pull-down transistor 16 is configured as an NMOS transistor. A tapping for the output signal DP or DM is provided between the two resistors 18. The output resistance and the output impedance of the output driver stages 12, 14 are thus determined substantially by the value of the resistors 18.
In order to be able to match the output impedance of a driver stage as accurately as possible to the employed power impedance and to compensate for the influences of variations due to the production process, temperature or fluctuations in the supply voltage, the driver stage may be configured to have an adjustable output impedance. For example, a driver stage having multiple inverter branches may have an adjustable output impedance that can be adjusted depending on control signals. Individual inverter branches are activated and deactivated to adjust the output impedance of the driver stage, such that the output impedance is lower the more inverter branches that are activated.
Multiple control signals that are generated depending on the output impedance of the driver stage to be adjusted are used. The number of driver elements connected in parallel (i.e. the number of inverter branches) may be adjusted such that the output impedance of a pull-up region and pull-down region corresponds to a reference resistance, or to a multiple of a reference resistance. A voltage that decreases over the pull-up region or the pull-down region is measured, and is compared by a comparator to a voltage drop over the reference resistor. The counting direction of a counter is controlled on the basis of the output signal of the comparator, whereby inverter branches are activated or deactivated depending on the count. If the count only alternates between two adjacent values, the output impedance of the pull-up region or of the pull-down region is matched with the reference resistor. However, a problem arises in this case that only interferences that occur in the frequency range above the clock pulse frequency of the control circuit can be filtered out. An interfering, low-frequency noise thus remains.
A voltage drop at the driver stage may be averaged by oversampling so that noise that occurs in the measurement of the output impedance can be suppressed. There is still however a problem that errors are produced due to a voltage misalignment of the comparator and due to the noise at the comparator inputs. In particular the supply voltage of the circuit is implicated as noise source. This noise affects both inputs of the comparator (i.e., the falling voltage at the driver stage), which is a measure of the output impedance, as well as the reference voltage. Overall, fluctuations of the output impedance from the desired impedance (i.e., the line impedance) may therefore arise that are greater than the minimal step width of the impedance adjustment.
Accordingly, there is a need for an improved accuracy in the adjustment of the output impedance of a driver stage so that the output impedance of the driver stage may be adjusted with an accuracy that ensures that the actual output impedance of the driver stage differs from a reference resistance by less than the minimal step width of the adjustment.